This invention relates to bonded structures and processes for preparing them. More particularly, it relates to a peelable bonded structure comprising a plurality of articles bonded through a coating. At least one of the articles is a metal article and the coating has a composite structure which facilitates peeling of the articles from one another when desired.
Bonded structures comprising a plurality of articles which are bonded together to such an extent that they can be peeled apart are often required in various fields, especially in the packaging of materials in vessels.
Sealing materials such as crown closures and other vessel closures, have heretofore been prepared by coating a surface-protecting paint onto a metal sheet, forming the coated metal sheet into a crown shell, cap shell or the like and bonding a packing material to the inner face of the formed article. In premium sales of bottled beverages and the like, systems have been adopted in which a prize or premium is given for a predetermined number of packings or a packing having a winning mark. In the production of crown closures or caps for use in such premium sales, it is essential that packings be readily peelable from crown shells or cap shells when desired. However, it is also necessary that packings be bonded to crown shells or cap shells to such an extent that separation does not occur during the preparation of such crown closures or caps, during their transportation, or during the step of sealing the bottles. Also, crown or cap shells should have adequate corrosion resistance to the contents of the vessel and they should be able to withstand such processing as crimping or roll-on. Also, from the sanitary viewpoint, it is not permissible to print the surface of a packing which directly contacts a beverage. In general, a packing per se is applied to the inner face of a crown shell or cap in the state where it has a certain flowability. Accordingly, it is desirable that when a packing is peeled off, a printing ink layer formed on the inner face of a crown shell or cap is transferred in the state adhering to the packing.
These special peeling characteristics are also required for cans having so-called "easy-open" -type mechanisms. For example, a can vessel having an easy-open end, which is prepared by forming one or more openings necessary for drinking through a can end-member composed of a surface-protecting coating-applied metal sheet and bonding a peel piece composed of an organic resin-coated metal foil or sheet over said opening or openings is known in the art. Also, in such easy-open end structures, excellent adhesion and sealing are required between the can end member and peel piece during storage, but when the can vessel is opened, the peel piece should be separable from the can end member without risk of breaking the peel piece.
In order to form a bonded structure of two articles which can be peeled from each other in the bonded area without breakage of the articles, it is necessary that the peel strength of the bonded region should be within a certain range, generally about 0.05 to 5 kg/cm. If the peel strength is lower than 0.05 kg/cm, peeling is undesirably found to occur before the articles are intentionally peeled. On the other hand, when the peel strength is greater than 5 kg/cm, it is generally difficult or impossible to effect peeling at the bonded interface without breaking the bonded articles.
Furthermore, in order to easily peel two bonded articles from each other at their bonded interface, it is necessary that peeling be facilitated in that portion of the bonded interface where peeling is initiated.
Accordingly, it is an object of the present invention to provide novel peelably bonded structures comprising a plurality of articles, in which at least one of the articles is composed of metal and which articles can be peeled apart when desired.
Another object of the invention is to provide a peelably bonded structure in which a plurality of articles are bonded together to such an extent that the bonded structure as a whole has a peel strength of between about 0.05 and 5 kg/cm or so that peeling can easily occur between the bonded articles in the peeling-initiating region.
Another object of the invention is to provide a vessel closure comprising a metal substrate of a vessel closure such as a crown shell or cap shell and a packing material peelably bonded to the metal substrate through a plurality of coating layers.
Another object of the invention is to provide an easy-open vessel comprising a can end member having an opening and a peel piece peelably bonded to the can end member through a plurality of coating layers.
Yet another object of the invention is to provide a process for producing the aforementioned peelably bonded structures.
These and other objects of the invention as well as a fuller understanding of the advantages thereof can be had by reference to the following detailed description, drawings and claims.